Process of annealing glass



Patented May 18, 1926.

`UNITED' STATES APATENT oFFlcE.

KENNETH M. HENRY, OF SAN FRANCISCO, CALIFORNIA, ASSIGNOR TO ILLINOIS- PACIFIC GLASS COMPANY, F SAN FRANCISCO, CALIFORNIA, A. CORPORATION OF CALIFORNIA.

' PROCESS OF ANNEALING GLASS.

Application led November 2'5, 1924. Serial No. 752,110.

This invention relates to the annealing of glass, and more particularly to the art of annealing as carried on in temperature-con trolled lehrs or muffles.

The annealing of glass as heretofore carried on has required a great'deal of time, and in the case of large bottles, `say of vegallon capacity, it has been practically impossible to properly anneal them in a lehr,

resort being had to annealing ovens, where the bottles have to beheld for several days. The object of my invention is to properly anneal glass articles, including large-sized bottles, in a lehr or temperaturemontrolled muffle, and to shorten the time required by a matter of several hours, or even days, de-

pending on the size and character of the article.

It will be helpful to an understanding of my process tobriefly describe some of the terms hereinafter used:

Annealing temperature or the upper limit ofthe annealing range is that temperature at which all existing stresses and L strains will be eliminated within two minutes after the glass is subjected thereto.

Annealing range is a Zone of about 270 degrees Fahrenheit below the annealing temperature.

Glass, according to the temperature to which it is subjected exists in three states, namely, viscous, plastic, and elastie Viscous state is that condition of the glass above the upper limits of the annealing range where the relaxation ofv stresses is. instantaneous.

Plastic state is that condition of the glass in the annealing range where the relaxation of stresses takes an appreciable length of time.

Elastic state is that condition of the glass below the annealing range where the relaxation of stresses takes an infinite length of time.

Temporary stresses are those imparted to the glass in the plastic and elastic states.` The temporary stress produced by a heating gradient is called a positive stress, and that produced by a cooling gradient is called a reverse stress.

Permanent stresses are those due to the imparting `of'a temperature gradient while in the viscous state. Permanent stresses may can be controlled at will, and all existing however, glass stress and strain can be removed, or stress and strain can be imparted that will remain vafter the annealing is completed.

Furthermore, for the-complete removal of all possible stress and strain, temperature and time are correlative factors. At any temperature (within the annealing range) there is a definite time interval necessary for removal of stress and strain, which I shall term the annealing time. Thus the annealing' time may consist of a brief period if the temperature maintained be high, or it will be a longer period if the temperature during the same be lower.

When a temperature gradient (rising or falling) is imparted to glass in the. viscous state, the molecules are free to move and do move toconform to the temperature gradient without the introduction of stress bei cause of such freedom. After this readjust- \ment, when the glass has cooled and the temperature gradient is removed, this condition is frozen and retained in the plastic and elastic states of the glass successively and is a permanent stress and strain. The same result, although partial in degree, may follow from a cooling gradient imparted to the glass inthe upper reaches of the plastic states. There is no sharp definition between. the plastic and the viscous state, and in the upper reachescf the former, the molecules of glass being somewhat mobile, may partially readjust themselves to conform to a coolinggradient and take on a change that becomes permanent stress and strain when the condition becomes frozen 'by the elastic states. of the glass.

The molecular readjustment to which reference has been made is called viscous yielding, or lost motion.

Under prior methods of annealing, these factors have not been properly observed, and it is the usual practice to bring the glass articles to a high temperature where all strains disappear in a reasonably short-time, andV In and by the present method, instead of accomplishing all of vthe heating in one stage, as in prior methods, provision is made for carrying on annealing'in two. or more stages, that is to say, the glass articles are rapidly heated to a relatively high point in the annealing range, bordering on the uper limit of the plastic state and the lower imit of the viscous state. Here the temperatureis maintained until all stresses and strains are removed, and tlien the temperature is rapidly lowered t0 a point within the annealing range and held there lbng enough to remove the reverse strains. caused by the rapid drop in temperature. Thereafter cooling can be carried on continuously and with progressively increasing rapidity), until the articles are at a temperature the annealing range, without danger of imparting reverse strains'. Thereby perfect elow ing period would alsoI be removed at this point. From this point the glass is cooled at a progressively increasing rate, which becomes very rapid as the lower limit of the annealing rano'e is reached. After the lower limit is passed, the cooling may be as, rapid as desired, saving only` that the temporary stress imparted by the rapid cooling may not be so great asx to overcome the tensile strength ofthe glass and thus result in kbi'eaka e.

In t e foregoing example but two annealing stages are used.- This `is not in i obedience to a. set rule, but is governed by the chemical and physical characteristics of the glass. In some glasses more than two stages of annealing may be required before the glass becomes suiiciently immobile and free from lost motion to ypermit a rapid cooling in -the inal stages of the process.

As for the tlieor of the several stages employed, let it be ere explainedthat the constant temperature at the high point in the annealing range would suffice to remove all stresses, but the cooling gradient from that point would necessari] be a slow one and critically controlled wit in very narrow limits, to avoid introducing strains anew b vviscous yielding. On the other hand, annea ing only at the lower reaches of the ran e, whence the risk of new strains would flic annealing can be eiected in a period of threeo heghghle; Would require a time COI that or four hours as against a period of twelve to seventy-two hours under prior practice.

In the accompanying drawing I show a chart illustrating-a temperature curve suitable for annealing ve-gallon bottlesof a particular character of glass. This curve will vary, depending upon the" nature of the glass and depending also in some instances upon the shape and size of the article. However, the necessary variations can readily be arrived at by those skilled in-the art, by means of preliminary tests.

Glass articles as they issue from the forming machine may be either hotter or colder is economically prohibitive. Therefore, my process removes the stresses at a High teinperature where the annealing time is short, then cools rapidly to a 'temperature still within the annealing range, where ordinarithan the annealin temperature as found for five gallons 0f 1hlui Content iS ShOWll in the particular g ass under treatment. In the 'accompanying drawing.l The vertically either event they are brought rapidl to spaced lines represent temperature in dethe proper annealin temperature, w ch grecs allrenheit, and the horizontally may be the upper limits of the plastic state spaced hheslilme h1 minutes- Although this or lower limits of the viscous state. When type 0f bottle has been manufactured for the annealing temperature is reached, the( many years, it has been impossible to anli eat is held constant until all of the stresses Deal 113 Consistently and properly, on account are removed. It follows thatthe higher the 0f its great weight and size. In fact, itis temperature to which glass is heated, the 110W known that'it would not be possible' shorter will be the time required to remove t0 anneal such a bottle in a lehr with goody the stresses. When the gradients have been commercial results without employing the removed the glass is cooled rapidly to a temprocess here disclosed. perature lower in the annealing range, and These bottles generally reach vtheannealthere again held constant for a suilicient ing apparatus at a temperature below the period of time tor remove the stresses im upper reaches of the annealing range. Imparted by viscous yielding during the coolmediately upon entering the lehr they are ing. Any stress that was inadvertently left. heated vrapidly to 930 degrees Fahrenheit,

in the glass at the end; of the rst anneal'- and ,held at this temperature for twelve 13 Fahrenheit at the rate of practice, as a factor of safety,

minutes. The temperature is then dropped to 875 degrees Fahrenheit as rapidly as the apparatus will permit. They are held at this temperature for a further period of twelve minutes. cooled to 835 degrees Fahrenheit at the rate of two degrees per minute, to 750 degrees five degrees per minute, to 500 degrees Fahrenheit at the rate of 10 degrees per minute, and thence to room temperature at will, with due regard for breakage from sudden shock. Due to the necessity for accurate control of temperatures, it is preferable to carry out this process in an electric lehr.

ln applying my process to glass of different physical or chemical nature, it is necessary rst to determine the proper annealing temperatures and times and the rate of cooling. This may be done by resorting to a few preliminary tests, rst to determine the upper limit of the annealing range fof Athe particular glass under consideration.

Thereafter varioustests are made to determine the temperature and time required for ferent points within the annealing range. lErom these. tests the lowest maximum temperature is chosen rwhose Iannealingtime is possible of commercial adaptation, prefer-- ably those temperatures with annealing times of from" fteen to twenty-,five minutes. ln the annealing time should be lengthened from three to five minutes, to insure against temperature fluctuations.

In the determination the initial cooling must lbe slow, since the lo-st motion is a function ofthe temperature and the rate of cooling. The amount of lost motion for identical cooling rates decreases by one-halfl for every 18 degrees Fahrenheit drop. Therefore, as the temperature lowers, the cooling rate may be increased, even within the annealing range, Awithout increasing the amount of lost motion. Regard should also be paid to the maximum thickness of the glass, since the stress is directly proportioned to the square of the thickness for unit gradient o r lost motion.

Since the process'is to be lcommercially applied in an electric lehr, the number of .changes in the cooling rate should be governed by the number of ly possible of cooling, either three or four. The length of time for cooling at a constant Successively they are` the removal of the maximum stress at difof the cooling rate,

sections economicalrate is determined by the length of the heating element section, and the speed of the conveyor within the lehr.

ln and by the present process, a glass article can be completely annealed from every possible degree and variety of stress and strain at a high temperature, and then by re-annealing at a lower temperature the' stressimpartcd during the drop in temperature can be safely removed at a low temperature and the time of cooling c..nsiderably shortened.

.ln all known processes heretofore used, it was impossible to handle different weights and sizes of glass varticles Without changing the annealing cycle. In other words, a curve designed for small ware would not anneal` the larger sizes. The process here set forth will correctly anneal articles of any size or weight without change, provided only that the glass be'of the same chemical and physical nature. y

My process provides a factor of safety and uniformity hitherto unobtainable in commercial practice. i

' inally, the process naturally economizes both space and time. This is particularly emphasized in the treatment of the larger sizes of, glass ware. For example, the fivegallon bottle already cited is perfectly and certainly annealed and ready for packing in three hours, whereas no less than twelve hours has been necessaryA `for the imperfect annealing hitherto attained in lehrs, and in the case of ovens three to four days were required. t

The present method differs from that disclosed in my Patent Number 1,540,264, issued June 2, 1925, in that the former contemplated partial removal of stress and strain at annealing temperaturesand neutralizing the remaining stresses and strains by the cooling curve or gradient; whereas in the. present method all stresses and strains are removed at high annealing temperatures, whereupon the vtemperature is suddenly lowered and re-annealing carried on at this lower temperature.

What I. claim as new and desire to secure by Letters Patent is:

1. lin a process of annealing glass articles in a temperaturefcontrolled lehr or muflie, the steps of ,subjecting the glass-ware to a. plurality of annealing stages, first at a high temperature and thereafter at a lower temperature, all within the annealing range.

2. ln a process of annealing glass articles in a temperature-controlled lehr or inutile, the steps' of subjecting the glass-ware to uniform temperature for a relatively short time within the upper reaches of the annealing range, lowering the temperature a substantial amount with appreciable rapidity, and re-annealing at said reduced temperature.

3. ln a process for the annealing'of glass articles in a temperature-controlled lehr or Inutile, the steps of first annealing at a relatively high temperature, rapidly reducing the temperature to a point still Within'the annealing temperature, and re-annealing at such lower temperature, and thereafter continuously reducing the temperature to a pointbelow the annealing range.

4. A process of annealing glass articles in a leliror'the like, which 4consists in irst 'bringing the lass-ware to :a temperature Suiiiciently hi stresses in the most economical time, lowering the temperature a substantial amount as rapidly ias possible, maintaining this .low-k ered temperature for a period of time suicient tore-move anyjreverse strains imparted d-urino". theurapid ,fall of. the itempeiatul'e',

and t reafter 'continuously andl withv progressively increasing rapidity lowering the temperature to a point below the lower limit l of the annealing range. 4

5. A process of annealingiglass articles in a; lehr or' the like, which consists in 'bringing' the'glass-ware to a temperature within the fupper reaches of the annealing range, main'- `taining this temperature for a time suiii ciently longto remove all stresses, then rapidly lowering the temperature to a point still within thtJ annealing range, maintaining this reduced temperature for a time sufficiently -long to remove al1 reverse strains caused by the sudden fall of temperature, and thereafter'continuously and with progressively increasing` rapidlty, lowering the temperature to a point below the annealingv range..-

. 6.' A process `ot' annealing glass articles in -a ,lehr or thelke, which consists inirst to cause removal 'of v allbringing thea'rticles to a temperature bor-V4 an'd t e lower limits of the. viscous stateo that particular glass, maintaining this temperature? suiciently-llong to remove all stresses, lowering the temperature with ap preciable'rapidity to a p oi'nt' withinV the-ann eallngE range, maintaining. this lowered temperature for atime suii'ciently 'long 'to' cause allreverse stressimparted by the sud-v den fal 'of temperature to be removed, and thereafter reducing t'he temperature with .temperature ra idly to approximately' 875 on ythe upper limits of the plastic'vv degrees Fahren eit, maintaining this latter l temperature for -ariod of approximately fifteen minutes. an thereafter reducing the temperature at approximately the fo-lowin'g rates: 2 degrees per minute-for twenty min u tes; 5 degrees per minute for the next seventeen minutes; and 10 degrees per mlnute for'the ensuing twenty-live minutes.

KENNETH M. HENRY. 

